The following references are believed to represent the state of the art:
U.S. Pat. No. 5,638,399 to Schuchman, et al.;
U.S. Pat. No. 7,065,213 to Pinder;
U.S. Pat. No. 7,496,198 to Pinder, et al.;
U.S. Pat. No. 7,584,495 to Hannuksela, et al.;
U.S. Pat. No. 7,613,112 to Jyske, et al.;
US Published Patent Application 2005/0041696 of Pekonen;
US Published Patent Application 2007/0002852 of Pekonen, et al.;
US Published Patent Application 2007/0002870 of Pekonen, et al.;
US Published Patent Application 2007/0002871 of Pekonen, et al.;
US Published Patent Application 2007/0147409 of Kallio, et al.; and
US Published Patent Application 2007/0288749 of Lee.